In a cathode ray tube (CRT), such as those used in color television sets or computer display monitors and the like, it is important that the electron beam used to excite, or illuminate, the CRT screen be properly aligned so as to strike or excite only the intended point or pixel (pixels).
However, CRT displays are prone to a problem called "image rotation" wherein the projected image appears rotated with respect to the horizontal axis of the screen. Image rotation is caused when the electron beam emitted from an electron gun of a CRT is deflected by an external magnetic field. One example of an external magnetic field is terrestrial magnetism. For example, as shown in FIG. 8, a CRT 1 is calibrated so as to project a normal image A on a screen, which is not rotated with respect to the horizontal axis of the screen, when it is manufactured. However when the display is used at a location having a different latitude from the latitude where the display was manufactured, in other words, the display is used at a location having different terrestrial magnetism from the terrestrial magnetism of where the display was manufactured, the image is rotated like the image B in FIG. 8. The amount of rotation may vary both in direction and magnitude depending on the geographic location of the display or the orientation of the display with respect to the external magnetic field.
Generally an image rotation compensating coil 10 is provided in a CRT which is to be used at a location having different terrestrial magnetism from the terrestrial magnetism of where the display was manufactured, as shown in FIG. 9(A), FIG. 9(B) and FIG. 10. The magnetic field generated by the image rotation compensating coil 10 offsets the terrestrial magnetism so that the rotated image B is corrected to the normal image A.
FIG. 9(A) and FIG. 9(B) show an example of a deflection yoke 2 having an image rotation compensating coil. A coil cover 3 is attached to a front part of the deflection yoke 2 used in a CRT 1 as shown in FIG. 9(A). The image rotation compensating coil 10 is wound around a coil cover 3 as shown in the figures. Two coil-winding frames 31 that are part of the coil cover 3 are provided at an outside surface of the coil cover 3 and are used to retain the image rotation compensating the coil 10. As shown in FIG. 9(B), a conductor 11 is wound between the coil winding frames 31 having a certain number of turns so as to form the image rotation compensating coil 10. Both terminals of the image rotation compensating coil 10 are connected to two pin terminals 32 which are provided at a side surface of the coil-winding frame 31. A certain electric current is supplied from a control means for controlling a direction or an amount of the electric current (not shown in figures) to the pin terminals 32. A magnetic field is thereby generated on the image rotation compensating coil 10.
Because the coil cover 3 includes the coil-winding frame 31, a mold for molding the coil cover 3 is complicated and the cost for manufacturing the coil cover 3 therefore becomes expensive.
Furthermore, when a plurality of deflection yokes 2 of the type shown in FIG. 9 are conveyed by a conveying belt 4 during manufacture (as shown in FIG. 11) and inclined at an angle 81, the deflection yoke may come in contact with the other deflection yokes. In this case, a part, such as a coil-winding frame 31, of the deflection yoke 2 may contact the image rotation compensating coil 10, and may damage the image rotation compensating coil 10. As a result, the image rotation compensating coil 10 may not operate properly.
FIG. 10 shows another example of a deflection yoke having an image rotation compensating coil 10. A plurality of coil retaining air-core parts 21 are provided at a front part of a deflection yoke 2. The coil retaining part 21 is formed by folding an end of a strip provided at the front part of the deflection yoke 2. Then, the image rotation compensating coil 10 is attached to the coil retaining part 21. In this case, both terminals of the image rotation compensating coil 10 are also connected to two pin terminals 32 (not shown in FIG. 10). An appropriate electric current is supplied from a control means (not shown in figures) for controlling the direction and amount of the electric current to the pin terminals 32.
In the manufacturer of the deflection yoke of the type shown in FIG. 10, it is necessary to have two steps including the first step for making an air-core image rotation compensating coil 10 and the second step for attaching the air-core image rotation compensating coil 10 to the deflection yoke 2. This increases the costs of manufacturing the CRT. Further an outer diameter of the deflection yoke shown in FIG. 10 is bigger than an outer diameter of the deflection yoke shown in FIG. 9(A) because of the coil retaining part 21. Such a design does not use an inside space of a television or a monitor efficiently. Furthermore, because the image rotation compensating coil 10 is exposed as shown in FIG. 10, it can easily contact other parts of the CRT causing damage the image rotation compensating coil 10. Accordingly, it would be desirable to have an image rotation compensation coil not having the problems associated with the prior art devices.